Characterization of Intermetallic Compound (IMC) growth in Cu wire ball bonding on Al pad metallization

As one of the alternative materials in chip interconnection, copper wire has become popular because of its lower cost and higher electrical conductivity than gold wire. Moreover it is known that long term reliability performance at high temperature of copper wire is better than that of gold wire because of slower Cu/Al intermetallic compound (IMC) growth than that of Au/Al intermetallics. However, majority of copper wire bonding development works has been focused on the material and/or process optimization and qualification so far, now it is time that we need to understand more on the Cu/Al IMC growth behavior to prevent IMC related failures in copper wire field application. So, in this paper, we aimed to generate Cu/Al IMC growth model based on the experimental result depends on high temperature storage (HTS) time and temperature and also tried to suggest Cu/Al IMC thickness guideline to minimize IMC degradation. In this experiment, Al pad chips were bonded with 99.99% purity of copper wire and Pd coated copper wire and some of them were encapsulated with epoxy mold compound. The samples were storaged at the temperature range from 150 C to 250 C upto 1000 hrs. IMC phase and thickness were analyzed by the help of SEM and EDX. In order to generate Cu/Al growth model, reaction rate (K) and activation energy (Delta Q) were calculated with above experimental results by using Arrhenius diffusion equation. Also, in order to investigate the Cu/Al IMC effect on the bondability, ball shear strength was measured and its result was correlated with IMC thickness. According to this paper, we could derive Cu/Al IMC thickness prediction model and suggest IMC thickness guideline that can minimize IMC failures.